A parallel universe, also known as an alternate universe, parallel world, parallel dimension, alternate reality, or alternative dimension, is a hypothetical universe co-existing with one's own, typically distinct in some way.[1] The sum of all potential parallel universes that constitute reality is often called the "multiverse." Another common term for a parallel universe is "another dimension", stemming from the idea that if the 4th dimension is time, the 5th dimension—a direction at a right angle to the fourth—is a direction into any of the alternate spacetime realities.
Fiction has long borrowed an idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus, and Valhalla are all "alternative universes" different from the familiar material realm. Plato reflected deeply on parallel realities, resulting in the worlds of Platonism, in which the upper reality is perfect while the lower (earthly) reality is an imperfect shadow of the heavenly equivalent.
One of the first science-fiction examples of a parallel universe is Murray Leinster's short story Sidewise in Time, published in 1934, which portions of alternative universes replace corresponding geographical regions in this universe. Sidewise in Time analogizes time to the geographic coordinate system, with travel along latitude corresponding to time travel moving through past, present and future, and travel along longitude corresponding to travel perpendicular to time and to other realities.
In modern literature, parallel universes can serve two main purposes: to allow stories with elements that would ordinarily violate the laws of nature; and to serve as a starting point for speculative fiction, asking the question "What if [event] turned out differently?". Examples of the former include Terry Pratchett's Discworld and C. S. Lewis's The Chronicles of Narnia, while examples of the latter include Harry Turtledove's Worldwar series.
A parallel universe—or more specifically, continued interaction between a parallel universe and our own—may serve as a central plot-point, or it may simply be mentioned and quickly dismissed, having served its purpose of establishing a realm unconstrained by realism.[clarification needed] Discworld, for example, only very rarely mentions our world or any other worlds, as Pratchett set the books in a parallel universe instead of in "our" reality to allow for magic on the Disc.
While technically incorrect, and looked down upon by hard science-fiction fans and authors, the idea of another "dimension" has become synonymous with the term "parallel universe". The usage is particularly common in movies, television and comic books and much less so in modern prose science fiction. The idea of a parallel world was popularized in comic books with the publication of The Flash No. 123, Flash of Two Worlds in 1961.
In written science fiction, "new dimension" more commonly—and more accurately—refer to additional coordinate axes, beyond the three spatial axes with which we are familiar. By proposing travel along these extra axes, which are not normally perceptible, the traveller can reach worlds that are otherwise unreachable and invisible.
In 1884, Edwin A. Abbott wrote the seminal novel exploring this concept called Flatland: A Romance of Many Dimensions. It describes a world of two dimensions inhabited by living squares, triangles, and circles, called Flatland, as well as Pointland (0 dimensions), Lineland (1 dimension), and Spaceland (three dimensions) and finally posits the possibilities of even greater dimensions. Isaac Asimov, in his foreword to the Signet Classics 1984 edition, described Flatland as "The best introduction one can find into the manner of perceiving dimensions".
In 1895, The Time Machine by H. G. Wells used time as an additional "dimension" in this sense, taking the four-dimensional model of classical physics and interpreting time as a space-like dimension in which humans could travel with the right equipment. Wells also used the concept of parallel universes as a consequence of time as the fourth dimension in stories like The Wonderful Visit and Men Like Gods, an idea proposed by the astronomer Simon Newcomb, who talked about both time and parallel universes; "Add a fourth dimension to space, and there is room for an indefinite number of universes, all alongside of each other, as there is for an indefinite number of sheets of paper when we pile them upon each other."[2]
There are many examples where authors have explicitly created additional spatial dimensions for their characters to travel in, to reach parallel universes. In Doctor Who, the Doctor accidentally enters a parallel universe while attempting to repair the TARDIS console in "Inferno". Douglas Adams, in the last book of the Hitchhiker's Guide to the Galaxy series, Mostly Harmless, uses the idea of probability as an extra axis in addition to the classical four dimensions of space and time similar to the many-worlds interpretation of quantum physics, although according to the novel they were more a model to capture the continuity of space, time and probability. Robert A. Heinlein, in The Number of the Beast, postulated a six-dimensional universe. In addition to the three spatial dimensions, he invoked symmetry to add two new temporal dimensions, so there would be two sets of three. Like the fourth dimension of H. G. Wells' "Time Traveller," these extra dimensions can be traveled by persons using the right equipment.
Perhaps the most common use of the concept of a parallel universe in science fiction is the concept of hyperspace. Used in science fiction, the concept of "hyperspace" often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel. Rationales for this form of hyperspace vary from work to work, but the two common elements are:
Sometimes "hyperspace" is used to refer to the concept of additional coordinate axes. In this model, the universe is thought to be "crumpled" in some higher spatial dimension, and that traveling in this higher spatial dimension, a ship can move vast distances in the common spatial dimensions. An analogy is to crumple a newspaper into a ball and stick a needle straight through: the needle will make widely spaced holes in the two-dimensional surface of the paper. While this idea invokes a "new dimension", it is not an example of a parallel universe. It is a more scientifically plausible use of hyperspace. (See wormhole.)
While the use of hyperspace is common, it is mostly used as a plot device and thus of secondary importance. While a parallel universe may be invoked by the concept, the nature of the universe is not often explored. So, while stories involving hyperspace might be the most common use of the parallel universe concept in fiction, it is not the most common source of fiction about parallel universes.
Time travel can result in multiple universes if a time traveller can change the past. In one interpretation, alternative histories as a result of time travel are not parallel universes: while multiple parallel universes can co-exist simultaneously, only one history or alternative history can exist at any one moment, as alternative history usually involves, in essence, overriding the original timeline with a new one. As a result, travel between alternative histories is not possible without reverting the timeline back to the original.
There are exceptions to the above, and an alternate history doesn't necessarily overwrite the old one. There are no rules written in stone regarding this. Modern ideas of time travel pose the idea of branching timelines, such as the 2009 Star Trek reboot and Avengers: Endgame. Technically, if a timeline is not explicitly stated to have been erased, it is still there.
Parallel universes as a result of time travel can serve simply as the backdrop, or it may be a central plot point. The Guns of the South by Harry Turtledove, where the Confederate Army is given thousands of AK-47 rifles and ends up winning the American Civil War, is a good example of the former, while Fritz Leiber's novel The Big Time where a war between two alternative futures manipulating history to create a timeline that results in or realizes their own world is a good example of the latter.
Subscribing to the many-worlds interpretation of Quantum Physics, alternative histories in fiction can arise as a natural phenomenon of the universe. In these works, the idea is that each choice every person makes, each leading to a different result, both occur, so when a person decides between jam or butter on his toast, two universes are created: one where that person chose jam, and another where that person chose butter. The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time", is used to allow characters to pass through many different alternative histories, all descendant from some common branch point.
Often, worlds that are more similar to each other are considered closer to each other in terms of this sidewise travel. For example, a universe where World War II ended differently would be "closer" to us than one where Imperial China colonized the New World in the 15th century. H. Beam Piper used this concept, naming it "paratime" and writing a series of stories involving the Paratime Police who regulated travel between these alternative realities as well as the technology to do so. Keith Laumer used the same concept of "sideways" time travel in his 1962 novel Worlds of the Imperium. More recently, novels such as Frederik Pohl's The Coming of the Quantum Cats and Neal Stephenson's Anathem explore human-scale readings of the "many worlds" interpretation, postulating that historical events or human consciousness spawns or allows "travel" among alternative universes.
Universe 'types' frequently explored in sidewise and alternative history works include worlds whose Nazis won the Second World War, as in The Man in the High Castle by Philip K. Dick, SS-GB by Len Deighton, Fatherland by Robert Harris, and Earthside by Dennis E. Taylor, and worlds whose Roman Empire never fell, as in Roma Eterna by Robert Silverberg, Romanitas by Sophia McDougall, and Warlords of Utopia by Lance Parkin.
The concept of counter-Earth might seem similar to a parallel universe but is actually a distinct idea. A counter-Earth is a planet that shares Earth's orbit but is on the opposite side of the Sun, and thus cannot be seen from Earth. There would be no necessity that such a planet would be like Earth in any way, although typically in fiction it is practically identical to Earth. Since Counter-Earth is within our universe and the Solar System, reaching it can be accomplished with ordinary space travel.
Convergent evolution is a biological concept whereby unrelated species acquire similar traits because they adapted to a similar environment and/or played similar roles in their ecosystems. In fiction, the concept is extended whereby similar planets will result in races with similar cultures and/or histories. Again, this is not a true parallel universe since such planets exist within the same universe as our own, but the stories are similar in some respects. Star Trek frequently explored such worlds, in episodes including "Bread and Circuses", "The Omega Glory", and "Miri".
The 2017 episode of British science fiction television programme Doctor Who, "The Doctor Falls", explains the different origins of the Cybermen as parallel evolution, due to the inevitability of humans and human-like species attempting to upgrade themselves through technology; this perspective resolves continuity differences in the Cybermen's history.
Convergent evolution may also be due to contamination. In this case, a planet may start out differently from Earth, but due to the influence of Earth's culture, the planet comes to resemble Earth in some way. Star Trek also frequently used this theory as well, for example, in "Patterns of Force" and "A Piece of the Action".
Simulated realities are digital constructs featured in science fiction such as The Matrix or The Thirteenth Floor which can parallel ours very closely.
It is common in fantasy for authors to find ways to bring a protagonist from "our" world to the fantasy world. Before the mid-20th century, this was most often done by hiding fantastic worlds within unknown, distant locations on Earth; peasants who seldom, if ever, traveled far from their villages could not conclusively say that it was impossible that an ogre or other fantastical beings could live an hour away. Characters in the author's world could board a ship and find themselves on a fantastic island, as Jonathan Swift does in Gulliver's Travels or in the 1949 novel Silverlock by John Myers Myers, or be sucked up into a tornado and land in Oz. These "lost world" stories can be seen as geographic equivalents of a "parallel universe," as the worlds portrayed are separate from our own, and hidden to everyone except those who take the difficult journey there. The geographic "lost world" can blur into a more explicit "parallel universe" when the fantasy realm overlaps a section of the "real" world, but is much larger inside than out, as in Robert Holdstock's novel Mythago Wood.
However, increasing geographical knowledge meant that such locations had to be farther and farther off.[3] Perhaps influenced by ideas from science fiction, many works chose a setting that takes place in another, separate reality.[3] As it is now not possible to reach these worlds via conventional travel, a common trope is a portal or artifact that connects our world and the fantasy world together, examples being the wardrobe in C. S. Lewis' The Lion, the Witch and the Wardrobe or the sigil in James Branch Cabell's The Cream of the Jest.
In some cases, physical travel is not even possible, and the character in our reality travels in a dream or some other altered state of consciousness. Examples include the Dream Cycle stories by H. P. Lovecraft or the Thomas Covenant stories of Stephen R. Donaldson. Often, stories of this type have as a major theme the nature of reality itself, questioning whether the dream-world is as real as the waking world. Science fiction often employs this theme in the ideas of cyberspace and virtual reality.
In a few cases, the interaction between the worlds is an important element, so that the focus is not on simply the fantasy world, but on ours as well. Sometimes the intent is to let them mingle and see what would happen, such as introducing a computer programmer into a high fantasy world as seen in Rick Cook's Wizardry series, while other times an attempt to keep them from mingling becomes a major plot point, such as in Aaron Allston's Doc Sidhe. In that story, our "grim world" is paralleled by a "fair world" where the elves live and history echoes ours, where a major portion of the plot deals with preventing a change in interactions between the worlds.
The idea of a